Identify where a task can expand

Map the call path through the initial answer, tool selection, result return, repair, and final response. Identify stages that repeat conversation history or accept large tool outputs. Place controls where the application can actually enforce them. Asking a model to be economical does not establish a spending boundary.

Verify current billing categories, units, and configurable limits for the selected configuration. Use only documented and tested behavior in the estimate. Keep unknown output categories and failure accounting visible, start with a conservative scope, and avoid inventing rates or assuming an unverified cache discount.

Check remaining allowance before continuing

Bound total rounds, tool calls, input size, tool-result size, and elapsed time. Reserve room for in-flight calls as well as completed work before starting another step. Concurrent workers need atomic reservations or an equivalent coordination mechanism so they cannot all spend the same apparent remaining allowance.

An alert informs someone; an execution boundary must stop new calls. Explain progress and preserve completed results when the boundary is reached. Stopping expenditure does not reverse an external side effect. Reconcile an already-submitted action separately rather than treating budget exhaustion as proof that nothing happened.

Validate the boundary with controlled failures

Use local tools and small fixtures to simulate repeated tool requests, oversized results, concurrent reservations, and cancellation. Confirm that controls operate before the next billable call and that background workers do not continue after termination. Do not create a large real charge merely to demonstrate a limit.

Record the configuration and test date, then repeat affected checks after model, prompt, or tool changes. Distinguish application controls from service guarantees. Unless you can establish that a limit is enforced by the service, do not describe it as an absolute account spending cap. Reconcile actual charges with current rules and observed usage.

Sources & further reading

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